This invention relates to the non-destructive testing of material using ultrasonic waves.
As is known, a study of beams of ultrasonic waves reflected from the surface of materials yields valuable information about mechanical properties of the surface layers of the materials, more particularly about the variations in Young's modulus, such as those which may be induced by thermal treatment of the material.
In this connection, there is a known method which comprises directing a beam of ultrasonic waves on to the surface of the material at a variable angle of incidence, detecting the energy of the corresponding reflected beam and determining the critical angle of incidence at which the energy (or the amplitude of the reflected waves) passes through a minimum. Rayleigh waves appear at the critical angle. The critical angle can be used, if required, to calculate the propagation speed of the Rayleigh ultrasonic waves at the surface of the material. This speed, and therefore the measured critical angle, depend on the surface hardness of the material. Allowing for the thickness of the surface layer involved in the propagation of the Rayleigh waves, the method can be used e.g. to estimate, by means of the thus-determined hardness, the depth at which a steel component has been tempered during cementation or other thermal treatment.
In spite of its importance, this method is limited in its applications since the hardness measured at each point on the surface of the material always corresponds to an average thickness of the material involved in the measurement, and cannot therefore be used for the testing of variations in hardness with depth.